In recent years, biodegradable plastics have been actively developed as materials that can solve problems caused by the large burden of plastic waste on the global environment, such as harmful effects on the ecosystem, generation of harmful gas during combustion, and global warming due to a large amount of heat generated by combustion.
Particularly, carbon dioxide generated by combustion of plant-derived biodegradable plastics was originally present in the air. Therefore, combustion of plant-derived biodegradable plastics does not increase the amount of carbon dioxide in the atmosphere. This is referred to as “carbon neutral”, and is regarded as important under The Kyoto Protocol that sets targets for reducing carbon dioxide emissions. Therefore, active use of plant-derived biodegradable plastics is desired.
Recently, from the viewpoint of biodegradability and carbon neutral, aliphatic polyester-based resins have received attention as plant-derived plastics. Particularly, polyhydroxyalkanoate (hereinafter, sometimes referred to as PHA)-based resins have received attention. Among PHA-based resins, poly(3-hydroxybutyrate) homopolymer resins (hereinafter, sometimes referred to as P3HB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resins (hereinafter, sometimes referred to as P3HB3HV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resins (hereinafter, sometimes referred to as P3HB3HH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resins (hereinafter, sometimes referred to as P3HB4HB), polylactic acid, etc. have received attention.
However, the above-described PHA-based resins are slow in crystallization, and therefore require a long cooling time for solidification after heat-melting in mold processing, which causes problems such as poor productivity and temporal change in mechanical properties (especially, toughness such as tensile elongation at break) due to secondary crystallization that occurs after molding.
Therefore, blending of a PHA-based resin with an inorganic material such as boron nitride, titanium oxide, talc, lamellar silicate, calcium carbonate, sodium chloride, or metal phosphate has heretofore been proposed to improve crystallization. However, the blending with an inorganic material has many adverse effects on a obtaining molded article, such as reduction in tensile elongation and poor surface appearance, and is therefore poorly effective.
As other attempts to improve crystallization of the PHA-based resins, there are exemplified a method of adding a sugar alcohol compound derived from natural products, such as erythritol, galactitol, mannitol, and arabitol (Patent Literature 1), a method of adding a polyvinyl alcohol, chitin, or chitosan (Patent Literature 2), and the like. However, crystal nucleating agents having a substantially high effect have not yet been found at present.